U.S. Pat. No. 4,766,168 discloses hydroxy-containing polymers or oligomers as stabilizers for polyacetal. Microcrystalline and fibrous cellulose, both of which are non-melting at the melt processing temperature of polyacetal, are not specifically disclosed within this reference. Further, there is no recognition in this reference of the advantages that are obtained with the specific stabilizer combination used in the present invention.
U.S. Pat. No. 4,722,662 discloses a process for manufacturing oxymethylene copolymers stabilized against thermal degradation, comprising the heating of the copolymer in a medium containing water and a disubstituted cellulose ether to hydrolyze unstable oxymethylene ends and then separating said oxymethylene copolymer from said aqueous medium containing a disubstituted cellulose ether. Disubstituted cellulose ethers are known thermoplastics.
U.S. Pat. No. 4,111,887 discloses polyoxymethylene molding compositions exhibiting improved physical properties comprising an admixture of a polyoxymethylene polymer, a fibrous reinforcement which can include cellulosic fiber, and a polycarbodiimide.
U.S. Pat. Nos. 3,406,129 discloses melt blends of moldable cellulose polymer having free hydroxyl groups with up to 50% of acetal polymer and U.S. Pat. No. 3,406,130 discloses colloidal dispersions of such blends with certain solvents for the cellulose polymer, which compositions are alleged to have improved melt strength and elongation. U.S. Pat. No. 3,406,129 specifically teaches that the use of greater than 50 weight percent polyacetal is detrimental to the melt blend. The cellulose disclosed in these references is not microcrystalline cellulose as it is moldable, and therefore, meltable at the melt processing temperature of polyacetal. Further, there is no disclosure of the particular stabilizer system used in the present invention.
While some of the reference discussed above disclose incorporating certain particular celluloses into polyacetal compositions, none disclose the specific cellulose used in the compositions of the present invention, nor do any disclose that microcrystalline or fibrous cellulose, in combination with at least one of certain co-stabilizer compounds, improves the stability of polyacetal compositions